The present invention relates to a fuel spacer for a fuel assembly and, more particularly, an independent cell type fuel spacer for the fuel assembly.
A known fuel spacer for a fuel assembly incorporated in a boiling water type reactor has a structure, as for example, is disclosed in the Japanese Patent Laid-open (KOKAI) No. 59-65287 (65287/1984) and shown in FIG. 15. Referring to FIG. 15, the fuel spacer is composed of a plurality of tubular ferrules 41 in each of which a fuel rod is charged and these tubular ferrules 41 are arranged in a lattice structure, in which the adjoining ones are joined together by welding means, for example. A water rod is also charged in the tubular ferrule 41. As shown in FIG. 15, the ferrule 41 has inward projections 13b formed to its cylindrical wall, and a cutout 15 is formed to the ferrule wall and a pawl portion 16 is also formed so as to project in the cutout 15.
Also known are cylindrical ferrules 42 each having an octagonal cross section as shown in FIG. 16, in which each ferrule 42 has inward projections 43 formed to the cylindrical wall thereof and reference numeral 44 denotes a continuous loop spring.
Furthermore, there has been also studied a fuel spacer to be utilized for a fuel assembly in which a water rod has an outer diameter larger than an inner diameter of a cylindrical ferrule, such as that disclosed in the Japanese Patent Laid-open (KOKAI) No. 61-198096 (198096/1986).
Recently, there has been developed an analysis of a mechanism between a transition boiling generation and a shape of a fuel spacer of a fuel assembly. Namely, it has been found out that transition boiling is likely to be generated at a portion near the lower ends of a first or second fuel spacer from the upper side of the fuel assembly in an installed state at a time when a power of the fuel assembly approaches its critical power output (at present, seven or eight fuel spacers are disposed axially along the fuel assembly). The fuel spacer acts to deposit liquid drops of a coolant to a surface of a fuel rod on a downstream side of the fuel spacer by agitating two phase flows of steam and liquid, thereby making the thickness of a liquid film thicker. As a result, the critical power output increases, contributing in the increasing of thermal margin of the fuel, increasing the power density of a reactor and increasing the power generation capacity. Accordingly, it has been considered to increase the number of the fuel spacers to be arranged.
However, increasing of the number of the fuel spacers results in the increasing of fuel axial pressure loss. Because fuel spacers each have a relatively large local pressure loss due to rapid constriction or expansion of flow sectional area, and fuel spacer local pressure loss occupys a large portion of axial pressure loss. It provides a problem in stability of the fuel assembly and increasing the necessary lift of a pump for circulating a coolant in a core, which also results in an economical disadvantage.